Grate structure for solid fuel burners

ABSTRACT

In one aspect, a grate segment for a burner, comprises a substantially rigid body ( 30 ) having a dispersed array of cavities ( 38 ) filled with a refractory material. In another aspect, a solid fuel gasifier includes a housing ( 12 ) defining a gasifier chamber ( 14 ), and a solid fuel inlet ( 21 ), grate structure ( 22, 24 ) in the gasifier chamber. The grate structure includes base means, a plurality of agitator segments ( 32 ) positioned relative to the base means for reciprocating movement to agitate a fuel load in the gasifier, and means to support the base means and the agitator segments so that the latter reciprocate in a direction at a substantial inclination to both horizontal and vertical. In use of the gasifier a deep burden of fuel accumulates on the grate structure and the outward stroke of the reciprocation of the agitator elements is in the overall direction of flow of the fuel burden or generally oppositely thereto.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage of International ApplicationNo. PCT/AU01/01666 filed 21 Dec. 2001. This application claims thebenefit of Australian Application No. PR 2291, filed 22 Dec. 2000. Thedisclosure(s) of the above applications are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates generally to solid fuel burners and hasparticular, though not exclusive, application to a solid fuel burner ofthe type commonly referred to as a gasifier or gasifier combustor.

BACKGROUND ART

A gasifier generally includes a primary combustion chamber into whichsolid fuel is loaded on to a grate structure on which it is first driedand gasified via controlled primary combustion. The resultant gas isthen transferred into a secondary combustion chamber, which mayconveniently be a cycloburner, for secondary combustion to produce ahigh temperature relatively clean flue gas able to be used for a varietyof purposes, eg. power generation or heating. There is a small residueof inorganic matter.

A gasifier of the general type to which the present invention relates isdisclosed, for example in U.S. Pat. No. 4,716,842, and the technologygenerally is of particular interest in waste recycling, especially withan emphasis on so called “green power” generation. Specific solid fuelswhich may conveniently be gasified in this way include biological waste,agricultural byproducts, wood waste and biomass.

Problems experienced with gasifiers, as with many other high temperaturefurnace processes, include sintering or aggregation of solid fines, andthe difficulty of achieving thorough mixing of fuel and air/gas streams.It is thought by the inventor that these problems can be minimised overtime in the initial gasification process by appropriate design of thegrate structure and it is towards this preferred objective that thepresent invention is especially directed.

U.S. Pat. No. 5,680,824 discloses a grate structure in which moveablehollow grate plates with coolant passages are reciprocable betweenstaitonary grate plates.

The invention seeks to provide an improved grate construction forfurnaces generally, but especially for solid fuel gasifiers.

SUMMARY OF THE INVENTION

The invention accordingly provides, in a first aspect, a grate segmentfor a burner, comprising a substantially rigid body having a dispersedarray of cavities filled with a refractory material.

Preferably, the grate segment is one of two or more (more preferablymultiple) complementary, generally planar grate segments for forming agrate assembly, in which said cavities are open in a heat-exposedsurface of the assembly.

The refractory material is preferably a ceramic material.

Advantageously, the dispersed array of cavities is such that the gratesegment exhibits a generally honeycomb structure.

In one application, the solid fuel burner is a gasifier.

In a second aspect, the invention is directed to a solid fuel gasifier,including:

a housing defining a gasifier chamber;

grate structure in said gasifier chamber, including:

-   -   base means;    -   a plurality of agitator segments positioned relative to the base        means for reciprocating movement to agitate a fuel load in the        gasifier; and    -   means to support the base means and the agitator segments so        that the latter reciprocate in a direction at a substantial        inclination to both horizontal and vertical; and

a solid fuel inlet;

whereby in use of the gasifier a deep burden of fuel accumulates on thegrate structure and the outward stroke of said reciprocation is in theoverall direction of flow of the fuel burden or generally oppositelythereto.

The second aspect of the invention stems from an appreciation by thepresent inventor that an optimum outcome is achieved by adopting amobile grate structure which is a compromise between mere horizontalconveyance and the maximum agitation achieved by vertical disturbance ofthe fuel load, eg. in a fluidised bed burner. By the arrangement of theinvention, there is achieved a high degree of agitation of the fuel loadsimultaneously with a controlled but relatively slow conveyance of theload.

Preferably, a grate structure according to the second aspect of theinvention comprises respective substantially fixed grate sectionsforming the base means, and moveable grate sections forming the agitatorelements. These moveable sections reciprocally slide over the fixedsections. There may be multiple pairs of said fixed grate sections andsaid moveable grate sections.

Preferably, means is provided to admit air to the fuel load via passagesand/or port means in the fixed grate sections. The air admission meanspreferably opens into the fuel burden in the end surfaces of the fixedgrate sections.

Preferably, the gasifier has a solid fuel inlet positioned generallybehind the grate structure(s) relative to a general direction ofprojection of the grate structure and overall direction of flow of thefuel, whereby in use of the gasifier a deep burden of fuel accumulateson the grate structure and the outward stroke of said reciprocation isin the overall direction of flow of the fuel burden. Preferably thissustained reciprocating movement is effective to agitate the fuel andenhance its combustion, and to steadily and slowly stroke or convey italong the chamber.

Preferably, the grate structure is arranged as a plurality of banks ofagitator elements, including a first bank with respect to the fueldelivery means that serves as a preheater and drying grate, and a secondand optional subsequent banks that provide for gasification of the fuelvolatile matter and fixed carbon oxidation.

Preferably, the preheater grate is arranged so that the agitatorsegments reciprocate in a direction more steeply inclined to thehorizontal than that for the gasification grate(s) and the optionalsubsequent banks are less steeply inclined than the second grate forgasification.

In a preferred embodiment of the second aspect of the invention, eachgrate section is formed from grate segments in accordance with the firstaspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic vertical longitudinal section of a solid stategasifier incorporating an embodiment of both aspects of the invention;

FIG. 2 is an enlargement, with additional detail, of the grate structureforming part of the gasifier illustrated in FIG. 1;

FIG. 3 is a further enlargement of part of FIG. 2;

FIG. 4 is a plan view of one of the fixed grate segments;

FIGS. 5, 6 and 7 are respectively an end elevation, a side elevation anda cross section on the line 7 to 7 of FIG. 4;

FIGS. 8 to 10 are views corresponding to FIGS. 5 to 7 but for areciprocating grate segment; and

FIG. 11 is a fragmentary isometric view of a modified form of the gratestructure.

DESCRIPTION OF PREFERRED EMBODIMENTS

The solid fuel gasifier 10 of FIG. 1 (which includes a figure of a man11 to provide a dimensional context), includes an outer housing 12 abouta gasification chamber 14 extending from a fuel delivery auger 9 towardsa separately walled cycloburner 16 that defines a secondary combustionchamber 17. A grate structure 20 includes a preheating grate 22 adjacentthe delivery end of auger 9, and, downstream in the overall direction offlow of the solid fuel, a gasifier grate 24. The two grates 22, 24 eachinclude stepped pairs of fixed 30 and reciprocating 32 grate segmentsand are linked by a near vertical grate 40 with multiple angled andcontrollable openings 42 for admission of combustion air from below thegrate structure into the fuel load above. It will be seen that the solidfuel inlet 21 into chamber 14 from auger 9 is generally behind the gratestructure 20 relative to the general direction of projection of thegrate structure and the overall direction of flow of the solid fuel.

In general, solid fuel delivered via auger 9 accumulates as a deep loador burden 70 on the grate structure while being dried and preheated ongrate 22 and gasified above grate 24. Combustible gas (syngas) is drawnthrough a transverse slot port 19 into chamber 17, from one end 15 ofwhich is recovered combusted flue gas or syngas useable for subsequentheating or power generating purposes. Inorganic solid residue or ashthat falls over the downstream end of grate 24 is directed by a baffledevice 50 into an ash grate 52 arranged on the heated floor 13 of thehousing. The ash is gradually agitated along the floor 13, and movedwhile remaining carbon is oxidised, for transverse removal and recoveryby conveyor 54.

An overhead water-cooled refractory lining 56 is suspended from the roof11 of housing 12 and merges into the wall structure of cycloburner 16.Lining 56 also defines one edge of slot port 19 through whichcombustible gases pass from the primary chamber 14 to the secondarychamber 17. Roof 11 supports an emergency exhaust stack 58.

Grates 22, 24 are of generally similar construction and essentiallydiffer only in their exact inclination, as will be discussed in duecourse. Each fixed grate segment 30 is of generally flat plateconfiguration, is, for example, about 0.3 to 1 metre front to rear, andis formed from a base plate 33 (FIGS. 4-5) and a top plate 34. Plates33, 34 may be clamped together by multiple screws 35 applied from belowand from the front. The width of the plate is set to match that ofchamber 14. The two plates 34, 35 which are cast in metal alloy or asuitable ceramic, preferably a heat resistant/high temperature alloy, soas to form a substantially rigid body, have complementary channels 36 a,36 b that define an open-ended passage 36 within the grate segment thatis a square U when viewed in plan. During operation of the gasifier,combustion air is supplied to passage 36 and from there is ejectedthrough the front face of the grate segment via plural nozzles 37 (bestseen in FIG. 11). The air delivered via nozzles 37 may be or includerecycled gases from the cycloburner or elsewhere.

Top plate 34 is also punctuated by a dispersed regular array ofhexagonal cavities 38 which open in the top surface of the plate, aregenerally shaped and arranged honeycomb fashion, and are filled with anappropriate high temperature refractory material, eg a suitable ceramic,in the finished grate segment. Cavities 38 are preferably on a 10-100 mmsquare array, with 5-20 mm alloy wall thickness.

Air passages 36 communicate with transverse air manifold ducts 39 (FIG.2) at the rear of each fixed grate segment and these ducts 39 arelinked, as diagrammatically depicted in FIG. 2, to an air supply conduit39 a.

Each reciprocating grate segment 32 is also of two part construction, inthis case comprising a front section 53 of generally right angular formand a main body 54 also having refractory (e.g., ceramic) filledhexagonal/honeycomb cavities 58 (FIGS. 3 and 5).

Water passages 80 may be located about the periphery of the lowersurface of each grate segment in order to maintain the segment at atemperature whereby overheating does not occur. However, the particularpreferred design of grate segment employed is able to operate at higherthan conventional temperatures.

It will be seen from FIGS. 1 and 2 that, in each grate set 22, 24, thefixed grate segments 30 are staggered in stepped fashion, being alignedat a uniform angle to the horizontal so as to slightly overlap, and thatthe reciprocating segments 32 are slidable between and atop the fixedsegments 30. The angle of inclination can vary between horizontal and60° to horizontal outward and upward, but preferably at least 10°, andis steeper (about 40° versus about 30°) in preheater grate 22 relativeto gasifier grate 24. This is because the typically wet fuel grate 22has a greater angle of repose and larger particle size than the fuel ongrate 24.

Referring to FIG. 2, in a larger installation, a further grate 26 may beprovided downstream of grate 24, optionally having a still smallerinclination to the horizontal. This third grate zone may enhancepost-reduction of the residual ash component from grate 24. Typically,this third grate 26 would be separated from grate 24 by a second stepgrate 41—preferably higher than step grate 40.

Grate segments 32 are reciprocated jointly over a stroke extendingapproximately between the front edges of the adjacent fixed segments:the fully retracted position is depicted by heavy lines in FIG. 1 andthe fully extended position by light lines. The outward or forwardstroke of the reciprocation is in the overall direction of flow of thefuel burden. The stroke is typically in the range 100-500 mm, and thestroke period is preferably 10 secs or more eg. in the range 15 secs to10 mins. A suitable arrangement for effecting reciprocation is shown inthe drawings. Each set or bank of grate segments has a pair of driveshafts 62 with respective tooth sectors 63. These engage with racks 64carried by a respective drive arm 66 which also carries the coolingwater. A cross-link 68 is coupled to respective drive arms 66 a for theother segments so that the grate segments reciprocate in unison.

The drive gear sectors are arranged to be driven in a first directionwhich causes the drive arms to move longitudinally relative to thelongitudinal axis of the arms so as to extend the moveable gratesegments relative to the fixed grate segments. Reversed movement of thedrive sectors causes a retraction of the moveable grate segments 32.Thus, reciprocating movement of the drive sectors causes the moveablegrate segments to move back and forth relative to the fixed gratesegments 30.

In operation, solid fuel is delivered by auger 9 to the combustionchamber 14. A bed of the fuel forms atop grates 22, 24 in theapproximate volume indicated by the dot-dash line 70 in FIG. 1. The bodyof solid material is predried on grate 22, while gasification orpyrolysis takes place above adjacent grate 24. Air for combustion (whichmay be wholly or partly recycled gases from the cycloburner and/or theprimary combustion chamber) is delivered into the fuel body both throughopenings 42 in step grates 40 and via air nozzles 37 in the front facesof fixed grate segments 30. Sustained reciprocating movement of moveablegrate segments 32 is effective to agitate the fuel and enhance itscombustion, and to steadily and slowly stroke or convey it along thechamber. There is also a degree of reverse rolling agitation of thematerial in the fuel burden, further enhancing the gasificationreactions. The fines in particular are mixed and agitated but withoutsignificant expulsion of airborne particles. The result is an optimumfuel load profile. The height of the fuel pile in the drying zone atgrate 22 is regulated by the speed of the grate relative to the fuelfeed auger speed, and the height of grate 24.

A typical profile of fuel in the various zones can be ascertained fromthe representation 70 in FIG. 1. As can be seen, the greatest thicknessof the fuel pile in any zone is located over the movable grates 32 mostadjacent the upstream step grate 40. It is believed that this fuel pileprofile provides good gasification zones. In furnaces, fuel profiles anddepths would typically be quite different from that illustrated.

The openings 42 in step grate 40 may preferably increase in size towardsthe lower edge of the step grate, or may only extend over a portion, eg.a lower portion, of the step grate. There may be one or more air boxesbehind the step grate that preferably taper to a cross-section at theupper side matching the adjacent fixed grate segment 30. The air box maythen supply controlled air through this fixed grate segment.

FIG. 11 illustrates a modified form of the grate structure, with likepairs indicated by line reference numerals.

It is thought that the design of the grates in the preferredconstruction described above is effective in reducing maintenancerelative to that conventionally required with the high temperatures andreducing atmosphere. The ceramic filled honeycomb cavity configurationenables a significantly higher temperature of the fuel mass on the gratesurfaces, relative to that achievable with conventional steel surfacedgrates, and thereby allows more efficient combustion and gasification.

1. A solid fuel gasifier comprising: a housing defining a gasifierchamber; a grate structure in said gasifier chamber comprising a base, aplurality of agitator elements positioned relative to the base forreciprocating movement to agitate a fuel load in the gasifier, means tosupport the base and the agitator segments so that the latterreciprocate in a direction at a substantial inclination to bothhorizontal and vertical with an outward stroke that is also upward; anda solid fuel inlet; said grate structure arranged as a plurality ofbanks of agitator elements comprising a first bank that serves as apreheater and drying grate, and at least a second subsequent bank thatprovides for gasification of the fuel volatile matter and fixed carbonoxidation; whereby in use of the gasifier a deep burden of fuelaccumulates on the grate structure, whereby said outward and upwardstroke of said reciprocation is in the overall direction of flow of thefuel burden or generally oppositely thereto; and wherein said preheaterand drying grate is arranged so that the agitator elements reciprocatein a direction more steeply inclined to the horizontal than that for thegasification grate(s).
 2. A solid fuel gasifier according to claim 1wherein said reciprocating movement is effective to agitate the fuel andenhance its combustion, and to steadily and slowly stroke or convey italong the chamber.
 3. A solid fuel gasifier according to claim 1 whereinfurther gasification banks are less steeply inclined than the secondgrate for gasification.
 4. A solid fuel gasifier according to claim 1wherein said grate structure comprises respective substantially fixedgrate sections forming the base, and moveable grate sections forming theagitator elements, which moveable sections reciprocally slide over thefixed sections.
 5. A solid fuel gasifier according to claim 4 whereinthere are multiple pairs of said fixed grate sections and said moveablegrate sections.
 6. A solid fuel gasifier according to claim 4 furtherincluding means to admit air to the fuel burden via passages and/or portmeans in the fixed grate sections.
 7. A solid fuel gasifier according toclaim 6 wherein said air admission means opens into the fuel burden inthe end surfaces of the fixed grate sections.
 8. A solid fuel gasifieraccording to claim 4 wherein each of said grate sections is formed fromgrate segments each comprising a substantially rigid body having adispersed array of cavities filled with a refractory material.
 9. Asolid fuel gasifier according to claim 8 wherein said refractorymaterial is a ceramic material.
 10. A solid fuel gasifier according toclaim 8 wherein said dispersed array of cavities is such that the gratesegment exhibits a generally honeycomb structure.
 11. A solid fuelgasifier according to claim 8 in which said cavities are open in asurface of the respective grate segment that is exposed to heat inoperation of the gasifier.
 12. A solid fuel gasifier according to claim1 wherein said solid fuel inlet feeds fuel into the chamber fromgenerally behind said grate structure relative to the overall directionof flow of the fuel.
 13. A solid fuel gasifier comprising: a housingdefining a gasifier chamber; grate structure in said gasifier chambercomprising: a base; a plurality of agitator segments positioned relativeto the base for reciprocating movement to agitate a fuel load in thegasifier; and means to support the base and the agitator segments sothat the latter reciprocate in a direction at a substantial inclinationto both horizontal and vertical; and a solid fuel inlet; wherein saidgrate structure is arranged as a plurality of banks of agitatorelements, including a first bank that serves as a pre-heater and dryinggrate, and at least a second subsequent bank that provides forgasification of the fuel volatile matter and fixed carbon oxidation,whereby the first bank and the second bank are linked by an uprightgrate such that a deep burden zone of fuel accumulates between the firstand second banks; and wherein said preheater and drying grate isarranged so that the agitator elements reciprocate in a direction moresteeply inclined to the horizontal than that for the gasificationgrate(s).
 14. A solid fuel gasifier according to claim 13 whereinsubsequent banks of agitator elements are linked by upright grates suchthat deep burden zones of fuel accumulate between the banks.
 15. A solidfuel gasifier according to claim 13 wherein further gasification banksare less steeply inclined than the second grate for gasification.
 16. Asolid fuel gasifier according to claim 13 wherein said grate structurecomprises respective substantially fixed grate sections forming thebase, and moveable grate sections forming the agitator elements, whichmoveable sections reciprocally slide over the fixed sections.
 17. Asolid fuel gasifier according to claim 16 wherein there are multiplepairs of said fixed grate sections and said moveable grate sections.